A(H3C3N3O3)(NO3) (A = K, Rb): Alkali-Metal Nitrate Isocyanurates with Strong Optical Anisotropy

Crystal clear: unveiling giant birefringence in organic-inorganic cocrystals

Coplanar groups with large anisotropic polarizability are suitable as birefringence-active groups for investigating compounds with significant birefringence. In this study, the organic coplanar raw reagent, o-C5H5NO (4HP), was selected as an individual complement. Utilizing the cocrystal engineering strategy, we successfully designed two cocrystals: [LiNO3·H2O·4HP]·4HP (Li-4HP2) and [Mg(NO3)2·6H2O]·(4HP)2 (Mg-4HP), and one by-product: LiNO3·H2O·4HP (Li-4HP), which were grown using a mild aqua-solution method. The synergy of the coplanar groups of NO3 - and 4HP in the structures resulted in unexpectedly large birefringence values of 0.376-0.522@546 nm. Furthermore, the compounds exhibit large bandgaps (4.08-4.51 eV), short UV cutoff edges (275-278 nm), and favorable growth habits, suggesting their potential as short-wave UV birefringent materials.

AYSO4F2 (A = K, Rb): [YO4F4] Polyhedra Enhancement of Birefringence in Non-π-Conjugated Sulfate Systems

A mild hydrothermal method was employed to successfully synthesize two new sulfate fluorides, namely, AYSO4F2 (A = K, Rb). They are isomorphic, and both contain [YO4F4] polyhedra and [SO4] tetrahedra in the structure. Theoretical calculations and experimental tests show that AYSO4F2 (A = K, Rb) have large band gaps (7.79 and 7.82 eV) and moderate birefringence (0.015 and 0.02 @ 546.1 nm), with significantly enhanced birefringence and band gaps as compared to that of the single alkali metal sulfates A2SO4 (A = K, Rb). Furthermore, theoretical calculations show that [YO4F4] polyhedra are the main reason for the band gap and birefringence enhancement. This work contributes to the advancement of structural chemistry in the field of rare-earth sulfates, offering a novel approach for the design of sulfates characterized by large birefringence.

Sr(NH2SO3)(NO3)·H2O: An Ultraviolet Nonlinear Optical Material Exhibiting Strong Second-Harmonic Generation Response and Sufficient Birefringence

Second-harmonic generation (SHG) response and birefringence stand as fundamental optical properties for nonlinear optical (NLO) materials. Up to now, engineering a strong SHG response and substantial birefringence in the sulfamate system has proven to be exceedingly challenging. A novel noncentrosymmetric compound, Sr(NH2SO3)(NO3)·H2O, has been meticulously designed by introducing the (NO3)- group characterized by significant polarizability anisotropy and hyperpolarizability into the polar non-π-conjugated (NH2SO3)- system. This compound exhibits a robust SHG response (5.2 × KDP), ample birefringence (0.077 at 546.1 nm), and a short ultraviolet (UV) absorption edge (290 nm). The linear and nonlinear optical properties can be attributed to the (NO3)- group and the (NH2SO3)- group. This study presents an effective approach that contributes to the design of more composite anionic UV NLO materials with excellent and balanced optical properties.

Cs2Mg(H2C3N3S3)4·8H2O: An Excellent Birefringent Material with Giant Optical Anisotropy in π-Conjugated Trithiocyanurate

The design of giant birefringence was performed by adjusting cations to make parallel and compact alignments of π-conjugated (H_xC₃N₃S₃)^x-3, where x = 1 and 2) groups with large polarizability anisotropy. Finally, the first mixed alkali/alkali-earth-metal trithiocyanurates, A₂B(H₂C₃N₃S₃)₄·nH₂O (A = K, Rb, Cs; B = Mg, Sr; n = 5-8, 12), were designed and synthesized successfully. Importantly, Cs₂Mg(H₂C₃N₃S₃)₄·8H₂O (III) and K₂Sr(H₂C₃N₃S₃)₄·5H₂O (IV) possess large birefringences of 0.580 and 0.194 at 800 nm, respectively, of III has the largest birefringence among all practical birefringent crystals, cyanurates, and hydroisocyanurates.

Two Carboxylate-Cyanurates with Strong Optical Anisotropy and Large Band Gaps

The first metal carboxylate-cyanurates, namely, K(H₃C₃N₃O₃)(HCO₂) (I) and Ba₂(H₂C₃N₃O₃)(CH₃CO₂)₃(H₂O) (II), which contain π-conjugated carboxylate and cyanurate groups, have been synthesized by hydrothermal methods. They crystallize in centrosymmetric space groups of P1̅ and P2₁/n, respectively. Compound I exhibits a novel three-dimensional (3D) structure based on a [K(H₃C₃N₃O₃)]⁺ cationic framework with 12-membered ring (12-MR) channels, and the (HCO₂)^- anions are located within the 12-MR channels. The [K(H₃C₃N₃O₃)]⁺ cationic framework is composed of K⁺ ions interconnected by H₃C₃N₃O₃ ligands. Compound II features a 3D network formed by [Ba₂(CH₃CO₂)₃]⁺ cationic double chains bridged by (H₂C₃N₃O₃)^- anions. The [Ba₂(CH₃CO₂)₃]⁺ cationic double chain is composed of (CH₃CO₂)^- anions and Ba²⁺ ions. Optical property measurements show that both compounds exhibit short ultraviolet cutoff edges (I, 208 nm; II, 218 nm) and wide band gaps (I, 5.43 eV; II, 5.20 eV). Importantly, K(H₃C₃N₃O₃)(HCO₂) (I) features a large birefringence of 0.285@532 nm due to the parallel alignment of π-conjugated H₃C₃N₃O₃ and (HCO₂)^- groups, indicating that K(H₃C₃N₃O₃)(HCO₂) (I) is a promising short-wave ultraviolet birefringent material. Detailed theoretical calculations elucidate that their excellent optical properties originate from the synergetic effect of both types of π-conjugated groups.

(C3N6H7)2SbF5·H2O Exhibiting Strong Optical Anisotropy from the Optimal Arrangement of π-Conjugated (C3N6H7)+ Groups

An antimony fluoride melamine birefringent crystal, (C₃N₆H₇)₂SbF₅·H₂O, was obtained by introducing the π-conjugated delocalized melamine and antimony trifluoride via a simple aqueous solution evaporation method. It features one-dimensional parallel [C₃N₆H₇]_∞ chains further connected by hydrogen bonds originated from [SbF₅]^2- groups with lone pairs. The experimental optical band gap (4.74 eV) allows it to be used in the ultraviolet region. The first-principles calculations suggest that (C₃N₆H₇)₂SbF₅·H₂O exhibits a large birefringence (∼0.38@550 nm), which is twice larger than that of the commercial CaCO₃ crystal. Therefore, introducing the fluoride into π-conjugated melamine may be a good tactic to obtain birefringent crystals with large optical anisotropy.

Directional Construction of New Nonlinear Optical Bifunctional Units through Molecular Engineering Design Inspired by the B3O7-Typed Configuration

Studies on new functional structural units with both large hyperpolarizability and high anisotropy are essentially important for finding high-performance nonlinear optical (NLO) materials and enriching the material systems. Under the guidance of the "structure-analogue" strategy, the work utilizes the molecular engineering approach to direct the construction of target units, BC₂N₅O₂ and B(C₂N₅)₂ units. The BC₂N₅O₂ unit with a highly analogous structure to the B₃O₇ group and its derivate B(C₂N₅)₂ unit with a configuration of B₅O₁₀ group are designed as NLO-active units. Furthermore, two compounds with these new NLO-active units, BC₂N₅H₆(OH)₂·H₂O (I) and B(C₂N₅H_6.5)₂(NO₃)₂ (II), are synthesized, successfully. These compounds exhibit excellent properties with second-harmonic generation (SHG) responses ranging from 0.5 to 5.9 times that of KDP and large birefringence (Δn_I = 0.181 @ 546.1 nm and Δn_II = 0.148 @ 546.1 nm). Theoretical calculations prove that the BC₂N₅O₂ and B(C₂N₅)₂ units make great contributions to the SHG effects and birefringence, which confirms that the BC₂N₅O₂ and B(C₂N₅)₂ units are novel NLO bifunctional units and could be excellent fundamental building blocks to construct amounts of novel NLO and birefringence crystals. Our studies would enlighten the research studies on biguanide complexes of boron.

Cd4InO(BO3)3: A New Nonlinear Optical Crystal Exhibiting Strong Second Harmonic Generation Effect and Moderate Birefringence

A new noncentrosymmetric cadmium indium borate, Cd4InO(BO3)3 (CIBO) has been successfully developed via a standard solid-state reaction. Its crystal structure was confirmed by the single crystal X-ray diffraction, which shows that CIBO belongs to the non-centrosymmetric and polar space group Cm. Its structure contains the distorted InO6 and CdOn (n = 6, 8) polyhedra, which link together by sharing an edge or corner to build a three dimensions framework with BO3 triangles accommodated in tunnels. Benefiting from the approximately parallel configuration of BO3 triangles, CIBO exhibited a strong second harmonic generation (SHG) effect (3 × KDP), and moderate birefringence of 0.077@1064 nm. Further optical and thermal characterizations suggest that CIBO possesses a wide transparent window and good thermal stability. Theoretical calculation reveals that the macroscopic SHG coefficients of CIBO results from the synergistic effect of the parallel arrangement of BO3 groups and d10 Cd2+ cation.

(C3N6H7)2SiF6·H2O: an ultraviolet birefringent crystal exceeding the intrinsic energy gap of an organic reagent

A new organic–inorganic hybrid compound (C3N6H7)2SiF6·H2O, in which [C3N6H7]+ groups are uniformly arranged under the regulation of SiF6 octahedra, shows highly polarization anisotropy and a breakthrough in the energy gap.

Two metal-free cyanurate crystals with a large optical birefringence resulting from the combination of π-conjugated units

Benefiting from the planar π-conjugated (H_xC₃N₃O₃)^x-3 (x = 0-3) groups, cyanurate crystals have recently become a research hotspot in birefringent materials. Herein, by combining the (H_xC₃N₃O₃)^x-3 (x = 0-3) group with the (CN₃H₆)⁺ cationic group, two metal-free cyanurates, GU(H₂C₃N₃O₃) (I) and GU₃(H₂C₃N₃O₃)₃(H₃C₃N₃O₃) (II), were obtained by the hydrothermal method. These compounds have wide band gaps (∼5 eV) and a large birefringence (∼0.40@400 nm), demonstrating their potential to be ultraviolet birefringent crystals. Moreover, first-principles calculations indicate that their large birefringence values originated from the synergistic effect of the (CN₃H₆)⁺ cations and (H_xC₃N₃O₃)^x-3 (x = 0-3) groups. These findings provide a new design strategy for exploring low-cost UV birefringent crystals with a large birefringence.

Two bismuth iodate sulfates with enhanced optical anisotropy

Two bismuth iodate sulfates crystallizing in the monoclinic space group P2₁/c, namely, Bi(IO₃)(SO₄) and CdBi(IO₃)(SO₄)₂, were synthesized via solvothermal reactions. Bi(IO₃)(SO₄) features 2D [Bi(SO₄)]⁺ layers, which are further linked by the IO₃^- groups to form a 3D network. CdBi(IO₃)(SO₄)₂ exhibits 1D [IO₃]^- chains built from IO₄^3- groups via corner-sharing and is the first example of a polyiodate sulfate as far as we know. These [IO₃]^- chains are interconnected by Bi³⁺ cations into [Bi(IO₃)]²⁺ layers parallel to the bc plane, whereas the neighbouring Cd²⁺ cations are interconnected by bridging SO₄^2- anions into [Cd(SO₄)₂]^2- layers, also parallel to the bc plane. These cationic and anionic 2D layers are held together through Bi-O-S bridges into a complicated 3D framework. Bi(IO₃)(SO₄) and CdBi(IO₃)(SO₄)₂ show wide band gaps of 3.91 and 4.03 eV and large birefringence values of 0.087 and 0.100 at 1064 nm, respectively. Our work indicates that the introduction of iodate group and lone pair cations, such as Bi³⁺, into metal sulfates can greatly enhance their birefringent properties.

Co-crystal AX·(H3C3N3O3) (A = Na, Rb, Cs; X = Br, I): a series of strongly anisotropic alkali halide cyanurates with a planar structural motif and large birefringence

Birefringent crystals with strong anisotropy are important components in modern optical devices. The newly discovered planar π-conjugated cyanurate group (H_xC₃N₃O₃)^x-3 (x = 0-3) has been demonstrated as an effective functional motif for improving birefringence in the ultraviolet region. Here, single co-crystals of alkali halide cyanurates, RbBr·(H₃C₃N₃O₃) (I), RbI·(H₃C₃N₃O₃) (II), and CsBr·(H₃C₃N₃O₃) (III) were synthesized by the ethanol solution method, and NaBr·(H₃C₃N₃O₃) (IV) was obtained via the solvent-drop grinding method. These four co-crystals feature a planar (H₃C₃N₃O₃) arrangement and exhibit wide band gaps (> 4.90 eV), tunable birefringence (Δn_exp∼ 0.124-0.256), and high thermal stability (156 °C-349 °C). In addition, first principles calculations were also carried out to evaluate the relationship between molecule density, spatial arrangement and optical birefringence, and suggested a great tailoring effect of the alkali metal and halogen species on regulating the optical anisotropy of co-crystal cyanurates.

Stepwise Li Substitution Induced Structure Evolution and Improved Nonlinear Optical Performance for Diamond-like Sulfides

One of the key scientific issues for exploration of novel infrared nonlinear optical (NLO) crystals is to obtain ones with good hybrid NLO behaviors. Herein, we report four diamond-like Ag-based sulfides via stepwise Li substitution of Ag in Ag₂ZnSnS₄, including Ag₂ZnSnS₄ (1), (Li_1.22Ag_0.78)ZnSnS₄ (2), (Li_1.58Ag_0.42)ZnSnS₄ (3), and Li₂ZnSnS₄ (4). With the increase of Li content, the sulfide's noncentrosymmetric crystal structure changes from tetragonal I42m for 1, to orthorhombic Pmn2₁ for 2 and 3, and to monoclinic Pn for 4. Accordingly, their NLO responses are improved along with the increase of Li content, viz. from non-NLO-active for 1, to non-phase-matchable for 2 and 3, and to phase-matchable for 4. Their optical band gaps also increase regularly. The relationship between their chemical compositions, crystal structures, and NLO activities is investigated by means of chemical structural analysis and theoretical calculation. This work offers a new systematic case for designing promising NLO-active compounds via rarely adopted cation's stepwise partial substitution and understanding the chemical composition-structure-NLO property relationship.

Zn(H2C3N3O3)2·3H2O: the first single-d10 transition metal based ultraviolet hydroisocyanurate crystal with large birefringence

The first single-d¹⁰ transition metal hydroisocyanurate crystal was designed and synthesized successfully with large band gaps and optical anisotropy.

K2Pb(H2C3N3O3)4(H2O)4: a potential UV nonlinear optical material with large birefringence

K₂Pb(H₂C₃N₃O₃)₄(H₂O)₄ (I) features a 2D [K₂PbO₈(H₂O)₄]^12- anionic layer and reveals a moderate SHG signal of approximately 2.6 × KDP.

2(C3H7N6)+·2Cl-·H2O: an ultraviolet nonlinear optical crystal with large birefrigence and strong second-harmonic generation

Large birefringence (Δn = 0.277 at 546 nm) and strong second-harmonic generation (SHG) intensity (4.3 × KDP) achieved a remarkable balance in a potential ultraviolet nonlinear optical crystal: 2(C₃H₇N₆)⁺·2Cl^-·H₂O. Theoretical calculations showed that planar benzene-like π-conjugated [C₃N₆] groups had important optical properties.